1. Field of the Invention
This invention relates to treating workpieces with beams and particularly to implanting ions in semiconductor wafers.
2. Description of the Prior Art
In ion implantation and other processes in which a beam of particles or radiation is directed at a workpiece, the beam must generally be moved across the workpiece in a controlled manner to spread the particle or radiation dosage evenly across the workpiece. In doping semiconductor wafers, a common technique is to move the wafers relative to a fixed beam along two orthogonal directions. The wafers are supported on a moving surface, which moves them at high speed along one direction and at a lower speed along an orthogonal direction. To achieve uniform doping density, it is conventional to measure the beam intensity, and vary the control speed accordingly, increasing the control speed when the intensity of the beam on the wafers increases and decreasing the speed when it decreases.
In the prior art, beam intensity has been detected in various ways summarized in U.S. Pat. No. 4,234,797 to Ryding.